High-specific-gravity epdm composition, dynamic damper made from the composition, tennis racket with the dynamic damper, and radiation-shielding material comprising the composition

ABSTRACT

A high specific-gravity EPDM composition consisting of a mixture of EPDM containing diene at less than 4.5 wt % and ethylene at not less than 58 wt % nor more than 80 wt % and having a Mooney viscosity ML 1+4  not less than 50 nor more than 170 at 125° C. and a powdery material, containing powder whose specific gravity is not less than 12 as a main component thereof, added to the EPDM at not less than 80 wt % nor more than 97.5 wt % of a whole amount of the high specific-gravity EPDM composition.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a high specific-gravity EPDMcomposition, a dynamic damper composed of the high specific-gravity EPDMcomposition, a dynamic damper-installed tennis racket, and radioactiveray shielding material composed of the high specific-gravity EPDMcomposition. More particularly, the high specific-gravity EPDMcomposition of present invention has improved moldability andweatherability to be used as the dynamic damper that is mounted onsports goods and the like and as the radioactive ray shielding materialof radiographic inspection appliances and the like.

[0003] 2. Description of the Related Art

[0004] The dynamic damper (vibration-damping material) is often used toreduce and relieve impacts and vibrations generated in sportingball-hitting goods and the like when they are used. For example, in thetennis racket, the dynamic damper is fixed to the racket frame thereofto resonate the dynamic damper with vibrations of the racket frame whenthe tennis racket hits a tennis ball. Thereby the vibrations generatedby an impact are relieved to reduce vibrations to be transmitted to aplayer's hand. In this manner, occurrence of tennis elbow is suppressed.

[0005] For example, the present applicant proposed a dynamic damper tobe installed on the tennis racket as disclosed in Japanese PatentApplication Laid-Open No.2002-085598 and No. 2002-048185. In theproposal, as the material for the mass-adding part of the dynamicdamper, the present applicant proposed a thermoplastic elastomer andchloroprene rubber in which lead and tungsten are dispersed. However,the lead which has been hitherto used in a large amount is inexpensivebut the use mode thereof and the use amount thereof are restricted toprevent environmental pollution.

[0006] There is a fear that a tennis player touches the dynamic damperor strikes the dynamic damper against others by mistake. In the casewhere the dynamic damper is formed of a hard material such as metal,there is a possibility that the tennis player is injured, which is notpreferable. Ordinary metals have a comparatively low specific gravity.Thus when they are used as the mass-adding part of the dynamic damper,the dynamic damper has a large volume, which disturbs the player duringthe use of the tennis racket. For appearance, it is preferable that thedynamic damper is small.

[0007] Therefore the tennis player desires a material not pollutingenvironment, soft, having a high specific gravity, and having acomparatively high strength so that it is not broken when it drops orstrikes against an object. For example, if the mass-adding part of thedynamic damper is sheet-shaped and its thickness is as thin as 0.6 mm toreduce its volume, it is necessary for the material having a highspecific gravity to be soft in such an extent that the material flowssmoothly in a molding die and have a strength in such an extent that itis not broken, even though it is thin and sheet-shaped. Because thedynamic damper is formed by combining the mass-adding part and theviscoelastic part with each other, it is necessary that the material hasadhesion to other materials.

[0008] For applications other than the dynamic damper, there is a demandfor the development of a material which has a high specific gravity(4-13) and is soft. Thus in recent years, a large number of rubbers andresinous materials having a high specific gravity are proposed.

[0009] For example, in Japanese Patent Application Laid-OpenNo.2000-27331, there is proposed a vibration-damping/sound insulationsheet to which slurry containing a large amount of a high specificgravity filler and an emulsion of rubber is applied.

[0010] In the field of shielding radioactive rays, in addition to aradioactive ray-shielding protection cloths used for medical purposes,only a predetermined portion is irradiated with a necessary amount ofthe radioactive rays in radiation therapy and measurement and portionwhich is not necessary is not irradiated. This is because there is ademand for prevention of destruction of normal cells and prevention ofexcessive exposure to radiation. As such, a radioactive ray shieldingmaterial is used in portions other than the portion to be irradiatedwith the radioactive rays. In fields other than the medical field, theradioactive ray shielding material is also used to shield theradioactive ray in inspections of food, inspections at a customhouse,and techniques of analyzing objects without destroying them.

[0011] Heretofore, a material in which lead, a lead compound, a leadalloy or antimony is blended in resin or rubber is generally used forthe radioactive ray shielding protection cloths. An acrylic plate or thelike has been used for comparatively weak radioactive rays. A tungstenand a plate made of an alloy thereof are also used as the radioactiveray shielding material.

[0012] As disclosed in Japanese Patent Application Laid-OpenNo.8-122492, there is proposed a radioactive ray shielding material madeof a resin containing a plasticizer in which tungsten is dispersed.

[0013] As disclosed in Japanese Patent Application Laid-OpenNo.10-153687, there are proposed vulcanized fluororubber and EPDM rubberboth containing tungsten dispersed therein and also chloroprene rubbercontaining tungsten dispersed therein.

[0014] However, the method of applying the emulsion to thevibration-damping/sound insulation sheet disclosed in Japanese PatentApplication Laid-Open No.2000-27331 is capable of forming a coatingfilm, but is incapable of forming molded products thick or complicatedin configuration.

[0015] In the radioactive ray shielding material disclosed in JapanesePatent Application Laid-Open No.8-122492, tungsten may settle in theprocess of drying and removing a solvent. Thus there is a room forimprovement in heat resistance and strength of the radioactive rayshielding material.

[0016] As described above, as the material for the radioactive rayshielding material, the vibration-damping/sound insulation sheet, thesoundproof material, and the like, there is a demand for development ofa material which is soft and has a high specific gravity. However, theproposed materials are all hard and difficult to handle and moreoverthere is a room for improvement in unpollutability, moldability, andprocessability.

[0017] In the radioactive ray shielding field, in the case where lead orits alloy is used, it is necessary to prepare a casting mold to processthe lead or its alloy into a predetermined configuration and dissolvethe lead or its alloy in the casting mold to cast it. A lead-castingwork is very costly because it is necessary to dissolve the lead or itsalloy and manufacture the casting mold. Further the dissolving the leador its alloy causes environment around a work place to deteriorate andaffect a human body adversely.

[0018] In the case where the radioactive ray shielding material is usedfor inspection of food or the like, when the radioactive ray shieldingmaterial is used in contact with a human body directly or indirectly,there is a fear that the lead which has separated from the radioactiveray shielding material contaminates environment. The melting point ofthe lead alloy is as low as 80° C. Thus in the case where theradioactive ray shielding material having the lead alloy is used for amedical purpose, a medical appliance or the like having the radioactiveray shielding material cannot be heated at about 100° C. although it isnecessary to sterilize it in hot water. Further the radioactive rayshielding material having the lead alloy cannot be used for a pipe orthe like of an atomic power plant or the like, because the pipe isrequired to have heat resistance at 200° C. As described above, in theradioactive ray shielding field, there is a demand for the developmentof a material which is high in specific gravity, the performance ofshielding radioactive rays, strength, moldability, and workability.

[0019] Chloroprene in which tungsten is dispersed is a little bad inweatherability. Thus in an outdoor exposure test (sunshine) which isconducted in a strict condition and in a sunshine weatherometer test ofexposing an object to ultraviolet rays, tungsten and additives areliable to bloom or whiten.

[0020] The vulcanized fluororubber disclosed in Japanese PatentApplication Laid-Open No.10-153687 is elastic, soft, strong to someextent, and superior in weatherability, but has a room for improvementin adhesion performance and in a process of combining other materialsand the fluororubber with each other. The EPDM rubber also disclosed inJapanese Patent Application Laid-Open No. 10-153687 has rubberelasticity and a high strength, is superior in weatherability, and moreadhesive than the fluororubber. Thus the EPDM rubber can be processed incombination with an adhesive agent or other materials, but has a roomfor improvement in moldability, processability, and weatherability inthe case where metal powder having a high specific gravity is dispersedin the EPDM rubber.

SUMMARY OF THE INVENTION

[0021] The present invention has been made in view of theabove-described problems. Thus it is a first object of the presentinvention to provide a high specific-gravity EPDM composition which issoft and high in specific gravity, moldability, processability,weatherability, and strength.

[0022] It is a second object of the present invention to provide adynamic damper that has a small volume, is high in vibration-dampingperformance, and is preferably attached to sports goods.

[0023] It is a third object of the present invenLion to provide a tennisracket on which a dynamic damper small and thin, not disturbing a playerin playing tennis, and superior in operability is installed.

[0024] It is a fourth object of the present invention to provide aradioactive ray shielding material which does not pollute environmentand is high inmoldability and processability, strength, and radioactiveray shielding performance.

[0025] To achieve the first object, as described in claim 1, the presentinvention provides a high specific-gravity EPDM composition consistingof a mixture of EPDM containing diene at less than 4.5 wt % and ethyleneat not less than 58 wt % nor more than 80 wt % and having a Mooneyviscosity ML₁₊₄ not less than 50 nor more than 170 at 125° C. and apowdery material, containing powder whose specific gravity is not lessthan 12 as a main component thereof, added to the EPDM at not less than80 wt % nor more than 97.5 wt % of a whole amount of the highspecific-gravity EPDM composition.

[0026] The molecular weight, the amount of the diene, and the amount ofthe ethylene differentiate the characteristic of the EPDM. The inventionof claim 1 is based on the present inventors' finding, made as a resultof experiments, that it is possible to obtain the high specific-gravityEPDM composition that is soft and high in specific gravity, moldability,processability, and weatherability, and strength by using the EPDM whichis specified to the above-described range in its diene amount, ethyleneamount, and Mooney viscosity and by adding the powdery materialcontaining powder whose specific gravity is not less than 12 as its maincomponent to the EPDM at a wt % in the above-described range.

[0027] The amount of the diene is less than 4.5 wt % (wt % of dienecomponent of entire material of EPDM) and preferably less than 3.5 wt %.

[0028] In an outdoor exposure test (sunshine) and a sunshineeatherometer test of exposing the high specific-gravity EPDM compositionto ultraviolet rays, the powdery material such as tungsten and the likeand an additive contained in the EPDM blooms in the case where the EPDMcontaining much diene is used. However, by reducing the diene amount ofthe EPDM to less than 4.5 wt %, it is possible to suppress generation ofthe blooming and improve the weatherability of the high specific-gravityEPDM composition.

[0029] To form the high specific-gravity EPDM rubber at a requiredstrength, it is important to optimize the range of the amount of theethylene and the range of the Mooney viscosity. It is preferable to usethe EPDM containing a large amount of the ethylene and having a highMooney viscosity.

[0030] Therefore the weight percentage of the ethylene of the entirematerial of the EPDM is set to not less than 58 wt % nor more than 80 wt% and favorably not less than 64 wt % nor more than 80 wt % and morefavorably not less than 64 wt % nor more than 70 wt %. Most favorablerange of the weight percentage of the ethylene is not less than 64 wt %nor more than 66 wt %.

[0031] If the EPDM contains less than 58 wt % of the ethylene, the EPDMhas a low strength. Thus in molding the high specific-gravity EPDMcomposition into a sheet after the powdery material is dispersed in theEPDM, the moldability of the high specific-gravity EPDM composition islow. On the other hand, if the EPDM contains more than 80 wt % of theethylene, the EPDM is hard. Therefore it is difficult to disperse thepowdery material such as tungsten and the like uniformly in the EPDM. Inthis case, in molding the high specific-gravity EPDM composition into athin sheet, its moldability is liable to deteriorate and a product suchas a dynamic damper formed by molding the high specific-gravity EPDMcomposition is hard. Thus the product strikes against a human bodystrongly.

[0032] The Mooney viscosity ML₁₊₄ of the EPDM at 125° C. is set to notless than 50 nor more than 170 and favorably to not less than 100 normore than 170 and more favorably to not less than 150 nor more than 165.

[0033] The Mooney viscosity is measured by the method provided by JISK6300 and used as an index indicating a viscosity. M of ML₁₊₄ is thefirst letter of Mooney and L of ML₁₊₄ is the first letter of L-typerotor. (1+4) of ML₁₊₄ means a preheating time period of one minute and arotation time period (four minutes) of a rotor.

[0034] If the Mooney viscosity is less than 50 at 125° C., the EPDM hasa low strength. Thus the mixture of the EPDM and the powdery materialsuch as the tungsten and the like dispersed therein has a lowmoldability.

[0035] On the other hand, the EPDM having the Mooney viscosity more than170 at 125° C. is hard. Thus it is difficult to accomplish uniformdispersion of the powdery material in the EPDM. Consequently the highspecific-gravity EPDM composition has a low moldability in molding itinto a thin sheet. Thus the product formed by molding the highspecific-gravity EPDM composition is hard and strikes against a humanbody strongly.

[0036] The molecular weight of the EPDM can be determined to someextent, based on the Mooney viscosity. The more the Mooney viscosity,the more the molecular weight of the EPDM. When the Mooney viscosity is50, the molecular weight of the EPDM is 300,000 - 400,000. When theMooney viscosity is 170, the molecular weight of the EPDM is about600,000.

[0037] The EPDM is mixed with the powdery material, containing thepowder having the specific gravity not less than 12 as its maincomponent, at not less than 85 wt % of the weight of the entire highspecific-gravity EPDM composition (total weight of EPDM, additives, andpowdery material) nor more than 97.5 wt %.

[0038] If the mixing amount of the powdery material containing thepowder having the specific gravity not less than 12 as its maincomponent is less than 85 wt % of the entire weight of the highspecific-gravity EPDM composition, the specific gravity of the entirehigh specific-gravity EPDM composition is not so high. In this case, inadding a necessary mass to the mass-adding part of the dynamic damper,it is necessary to make the mass-adding part large. Consequently thevolume of the dynamic damper becomes large.

[0039] On the other hand, if the mixing amount of the powdery materialcontaining the powder having the specific gravity not less than 12 ismore than 97.5 wt % of the entire weight of the high specific-gravityEPDM composition, the EPDM is incapable of covering the surface of thepowdery material. Consequently the strength of the high specific-gravityEPDM composition and its moldability are low.

[0040] In the case where the high specific-gravity EPDM compositioncontains the powdery material containing the powder having a specificgravity not less than 12 as its main component, it is possible toincrease the specific gravity of the high specific-gravity EPDMcomposition efficiently. Thus it is possible to reduce the volume of aproduct formed by molding the high specific-gravity EPDM composition.From the above view point, the powder having the specific gravity notless than 12 is contained in the powdery material at not less than 70 wt% and preferably at not less than 80 wt % of the weight of the entirepowdery material.

[0041] To disperse the powdery material in the EPDM favorably toincrease the strength of the high specific-gravity EPDM composition, theaverage particle diameter of the powder is favorably less than 50μm andmore favorably less than 20μm. The flow ability and moldability of thehigh specific-gravity EPDM composition can be increased by using powderhaving a small diameter, for example, less than 5 μm in combination withpowder having a large diameter, for example, more than 27μm.

[0042] Tungsten, a tungsten compound or a tungsten based alloy ispreferable as the powdery material containing the powder having thespecific gravity not less than 12.

[0043] Of metal materials, tungsten has a high specific gravity, isunharmful to a human body, inexpensive, and easily obtainable. Thus thetungsten can be preferably used. Since the tungsten has high specificgravity, it is preferable that the powdery material consists of thetungsten (100%). However it is possible to use the tungsten compound orthe tungsten based alloy. Alternatively a mixture of the tungsten, thetungsten compound, and the tungsten based alloy may be used. Thespecific gravity of the tungsten is 19.3.

[0044] The powdery material such as the tungsten or the like notchemically surface-treated by a coupling agent can be preferably used.For example, the powdery material can be preferably used in a physicallytreated state or in a powdered state.

[0045] If the powdery material is chemically surface-treated with thecoupling agent, the powdery material may bloom or the strength of thehigh specific-gravity EPDM composition may deteriorate.

[0046] To make the high specific-gravity EPDM composition soft, it ispreferable to add an appropriate amount of oil and the like to the EPDMas a softener.

[0047] In adding the softener to the EPDM, less than 150 wt % of thesoftener is added to 100 wt % of the EPDM.

[0048] The softener does not necessarily have to be added to the EPDM.But if not less than 150 wt % of the softener is added to 100 wt % ofthe EPDM, blooming is liable to occur in a weatherability test.

[0049] The specific gravity of the high specific-gravity EPDMcomposition of the present invention is set to not less than 4.5 normore than 13.1 and preferably not less than 5.0 no more than 9.5.

[0050] If the specific gravity of the high specific-gravity EPDMcomposition is less than 4.5, i.e., if the specific gravity thereof islow, the volume of the dynamic damper or the like is large. Thus thedynamic damper or the like disturbs a player in using a tennis racket.On the other hand, if the specific gravity of the high specific-gravityEPDM composition is more than 13.1, it is necessary for the highspecific-gravity EPDM composition to contain much powdery material,which makes it difficult to process the high specific-gravity EPDMcomposition.

[0051] It is preferable that the surface hardness of the vulcanized highspecific-gravity EPDM composition measured by the method specified byJIS K-6253 (tester durometer type A) is less than 90.

[0052] If the hardness of the high specific-gravity EPDM composition ismore than 90, it is so hard that it is difficult to mold the highspecific-gravity EPDM composition integrally with a material for theviscoelastic part constituting the dynamic damper. Supposing that thehigh specific-gravity EPDM composition satisfies other demandedcharacteristics, the lower the surface hardness, the better.

[0053] It is preferable that the tensile strength of the highspecific-gravity EPDM composition is not less than 3 MPa. If the ensilestrength thereof is less than 3 MPa, the high specific-gravity EPDMcomposition is liable to crack or break in molding it into the dynamicdamper or the like and when the dynamic damper or the like composedthereof is used. Supposing that the high specific-gravity EPDMcomposition satisfies other demanded characteristics, in the conditionthat it is not less than 3 MPa, the higher the tensile strength, thebetter.

[0054] The main chain of the EPDM consists of saturated hydrocarbon anddoes not contain a double bond. Thus even though the EPDM is exposed toan ozone atmosphere having a high concentration or a light-irradiatedenvironment for a long time, the main chain is not likely to cut. Thusthe high specific-gravity EPDM composition has a high weatherability. Inthe present invention, the kind and the like of the diene component ofthe EPDM is not limited to a specific one.

[0055] From the viewpoint of weatherability, the less the amount of thediene, the better. EPM containing no diene is superior to the EPDM inthis respect. However, since the EPM cannot be vulcanized with sulfur,it is necessary to vulcanize the EPM with peroxide. Since the speed ofthe vulcanization with the peroxide is lower than that of thevulcanization with the sulfur, the use of the EPM vulcanized with theperoxide causes workability to be low. Therefore the EPDM vulcanizedwith the sulfur is more favorable than the EPM in consideration ofweatherability and workability.

[0056] As the EPDM, it is possible to use both non-oil-extended typeconsisting of a rubber component and oil-extended type containing therubber component and oil. The weight of the oil of the EPDM of theoil-extended type added to the EPDM is treated as the added weight(amount of oil) of the softener.

[0057] The oil to be used as the softener is not limited to a specificone. Paraffin oil and naphthenic oil compatible with the EPDM arepreferable. In addition, it is possible to use known synthetic oil suchas mineral oil of aromatic series, oligomer of hydrocarbon series orprocess oil. As the synthetic oil, it is possible to use oligomer ofalpha olefin, oligomer of butane, and amorphous oligomer of ethylene andalpha olefin.

[0058] As the vulcanizing agent, sulfur is suitable because it has ahigh vulcanization speed and high workability. As an accelerator, it ispreferable to use 2-melcapto• benzothiazole, tetraethylthiuramdisulfide, zinc dibutyl• dithiocarbamate, and telluriumdiethyldithiocarbamate by appropriately combining them with each other.Thereby the rubber component can be efficiently cross-linked.

[0059] To achieve the second object, the dynamic damper of the presentinvention is composed of a viscoelastic part and a mass-adding part. Asthe mass-adding part, the high specific-gravity EPDM composition of thepresent invention is used.

[0060] As the mass-adding part, the high specific-gravity EPDMcomposition is used. Thus it is possible to make the volume of thedynamic damper of the present invention and its thickness small. It ispreferable to form both the mass-adding part and the viscoelastic partin the shape of a sheet and integrate them with each other, with bothparts layered on each other. The thickness of the mass-adding part isset to 0.3 mm-2.0 mm and favorably to 0.5 mm-1.0 mm. The addition of thethickness of the mass-adding part and that of the viscoelastic part isset to 3 mm-5 mm and favorably to 4 mm.

[0061] The thin sheet-shaped dynamic damper does not disturb a playerduring the use of a tennis racket and is unnoticeable in appearance anddoes not prevent from playing. Further since the dynamic damper is madeof a soft material, the player is not injured thereby. Furthermore thedynamic damper is so strong that it is not broken when it strikesagainst an object. Moreover because the dynamic damper has highweatherability, it can be used under the blazing sun.

[0062] As the material for the viscoelastic part of the dynamic damper,it is preferable to use the EPDM which is the macromolecular materialused for the mass-adding part or a macromolecular material similar tothe EPDM. In the case where a rubber material similar or same to theEPDM in a vulcanizing temperature and a vulcanizing time period is usedas the material for viscoelastic part, the EPDM for the mass-adding partcan be bonded there to by vulcanization in a die. That is, such a rubbermaterial is suitable for integral molding. The material for viscoelasticpart may be a foamed material. The material for viscoelastic part andthe material for the mass-adding part may be bonded to each other withan adhesive agent.

[0063] From the above-described standpoint, the EPDM can be preferablyused as the macromolecular material for the viscoelastic part. The EPDMcan be used singly or in combination of other components.

[0064] In addition, one of the following rubbers or a combinationthereof can be used for the viscoelastic part: natural rubber (NR),isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber(SBR), chloroprene rubber (CR), acrylonitrile-butadiene rubber (NBR),carboxylated butyl rubber, butyl rubber (IIR), halogenatedbutyl rubber(X-IIR), ethylene-propylene rubber (EPM), ethylene-polyvinyl acetaterubber (EVA), acrylic rubber (ACM, ANM), ethylene-acrylic rubber,chlorosulfonated polyethylene (CSM), chlorinated polyethylene (CM),epichlorohydrin rubber (CO), urethane rubber, silicone rubber, andfluorinated rubber and the like. Butyl rubber (IIR) is preferablebecause of the superior vibration absorption property.

[0065] As resin for the macromolecular material for the viscoelasticpart, thermoplastic resin and thermosetting resin are used. Thethermoplastic resin includes polyamide resin, polyester resin, urethaneresin, polycarbonate resin, ABS resin, polyvinyl chloride resin,polyacetate resin, polyethylene resin, polyvinyl acetate resin, andpolyimide resin. The thermosetting resin includes poxy resin,unsaturated polyester resin, phenol resin, melamine resin, urea resin,diallyl phthalate resin, polyurethane resin, and polyimide resin. Thethermoplastic resin is more favorable than the thermosetting resin inconsideration of moldability and because it can be recycled. It is alsopossible to use thermoplastic elastomers of styrene family, olefinfamily, urethane family, and ester family and the like.

[0066] The dynamic damper has a horizontal frame and a vertical framedisposed at both sides of the horizontal frame. Thus the dynamic damperis lattice-shaped. The horizontal frame and the vertical frame areintegral with each other or separately formed and bonded to each other.It is preferable that the horizontal frame is mounted on at least oneface of a racket frame in its thickness direction and that the verticalframe is mounted on both faces of the racket frame in its widthwisedirection.

[0067] It is preferable that the horizontal frame is bent in the shapeof a letter “U”, that one end of a bent portion of the horizontal framedisposed at both sides thereof is integral with the vertical frame orjoined therewith, and that the bent portion of the horizontal framedisposed at both sides thereof is installed on both faces of the racketin its widthwise direction. It is also preferable that the number of thehorizontal frames is not less than two and that the horizontal framesare mounted on the racket, with the horizontal frames sandwiching gutinsertion holes therebetween.

[0068] As described above, the horizontal frame and the vertical frameare continuous and integral with each other and lattice-shaped.Therefore in the dynamic damper-installed racket, the vertical frameresonates mainly with vibrations of the racket frame in a out-of-planedirection, whereas the horizontal frame resonates mainly with vibrationsof the racket frame in an in-plane direction, thus effectivelysuppressing vibrations in the out-of-plane direction and the in-planedirection. That is, because the horizontal frame and the vertical frameare disposed in the shape of a lattice, the dynamic damper has improvedvibration-damping performance, thus reducing impacts and vibrations.

[0069] The thickness direction of the racket means the directionvertical to the gut-stretched surface. The widthwise direction of theracket means the direction parallel to the gut-stretched surface.

[0070] In the case where the dynamic damper is formed monolithically inthe shape of a lattice, i.e., in the case where the vertical frame andthe horizontal frame are formed integrally with each other in the shapeof a lattice, the entire lattice resonates with the vibration of theracket frame in the in-plane direction, thus having an effect ofsuppressing the vibration in the in-plane direction.

[0071] The dynamic damper of the present invention may be formed bysetting a laminate of the material for the viscoelastic part and thematerial for the mass-adding part in a die and bonding them to eachother by vulcanization to mold them into the dynamic damper having adesired configuration. Alternatively the dynamic damper of the presentinvention may be formed by molding the high specific-gravity EPDMcomposition into a flat sheet and punching the sheet with a punchingblade to shape the sheet into a desired configuration and bonding theviscoelastic part and the mass-adding part to each other with anadhesive agent.

[0072] To achieve the third object, the present invention provides atennis racket having a dynamic damper installed on at least one portionof a head part surrounding a ball-hitting face of a racket frame or/andat least one portion of a throat part of the tennis racket.

[0073] A player can play tennis without caring about the dynamic dampermounted on the tennis racket, thus using the tennis racket with a highoperability. Further the player can play tennis comfortably withoutbeing annoyed by unpleasant vibrations or injury such as tennis elbowand the like.

[0074] Supposing that the top position of the ball-hitting facesurrounded with the head part of the racket frame is 12 o'clock byregarding the ball-hitting face as the surface of a clock, it isparticularly preferable that the dynamic damper is installed on at leastone portion of the head part in such a way that the dynamic damper isdisposed in an angular range of ±15° with respect to a three o'clockposition and a nine o'clock position. The three o'clock position and thenine o'clock position are maximum amplitude positions of the in-planevibration and that of the out-of-plane secondary vibration. Thereby thedynamic damper is capable of efficiently suppressing vibrations in thein-plane and out-of-plane directions without adversely affecting theoperability of the tennis racket.

[0075] Because a mass is applied to a wide portion (three o'clockposition and nine o'clock position) of the head part, the moment ofinertia around the grip becomes large. Thus when a ball collides with aportion of the ball-hitting face other than its center, the dynamicdamper prevents the rotation of the racket and reduces the degree ofburden to be applied to the player's elbow and the like.

[0076] From the viewpoint of balance, it is preferable that the dynamicdamper of the present invention is installed on the racket frame at leftand right positions symmetrical with respect to the widthwise center ofthe racket frame. But the dynamic damper-installing position is notlimited to these positions. A plurality of the dynamic dampers may bemounted on the racket frame at the left and right positions thereof. Itis preferable to form a concavity on the dynamic damper-installingposition.

[0077] It is preferable that the racket frame is made of a fiberreinforced resin. It is particularly preferable that the racket frame iscomposed of fiber reinforced prepregs layered one upon another in theshape of a hollow pipe. The racket frame may be made of materials otherthan the fiber reinforced resin by various manufacturing methods, forexample, metal and the like.

[0078] To achieve the fourth object, the present invention provides aradioactive ray shielding material composed of the high specific-gravityEPDM composition.

[0079] The radioactive ray shielding material of the present inventionis composed of the high specific-gravity EPDM composition, as describedabove. Therefore the radioactive ray shielding material does not polluteenvironment and has high moldability, processability, heat resistance,and strength, and radioactive ray shielding performance. Morespecifically, the performance of the radioactive ray shielding materialcan be improved by containing the powdery material having a highspecific gravity in the EPDM. Further by specifying the property of theEPDM to be mixed with the powdery material, the high specific-gravityEPDM composition is allowed to be elastic and have high moldability,processability, and durability.

[0080] Because the radioactive ray shielding material of the presentinvention is superior in processability, it can be cut easily withscissors and the like, and an opening can be easily formed. Thus theradioactive ray shielding material can be formed into variousconfigurations. Further because the radioactive ray shielding materialis flexible and is capable of making an elastic deformation, it can beeasily inserted into a gap which requires shielding of radioactive raysin the form of a sheet or the like in conformity to the configuration ofthe gap. Therefore it is possible to shield radioactive rays which haveleaked from a gap or the like. Further by utilizing the elasticdeformation of the radioactive ray shielding material, it can be easilymounted on an irregular portion and shield the radioactive rays.Furthermore since the radioactive ray shielding material has asufficient strength, it is not broken when it is inserted into the gapor the like and when it is processed. Thus it is possible to reliablyobtain desired radioactive ray shielding performance.

[0081] The higher the specific gravity of the radioactive ray shieldingmaterial is, the higher the radioactive ray shielding performancethereof is. In this respect, it is possible to increase the specificgravity thereof by mixing the powdery material and the like such astungsten (specific gravity: 19.3) with the EPDM at a higher mixing rate.Thus by setting the specific gravity of the radioactive ray shieldingmaterial larger than that of lead or a lead alloy (not less than 12),the radioactive ray-shielding effect provided by the radioactive rayshielding material is almost equal to or higher than that of the lead orthe lead alloy.

[0082] The gamma ray absorption coefficient (cm⁻¹) of tungsten is aboutone when the energy of the gamma ray is 1.5 MeV. The gamma rayabsorption coefficient (cm⁻¹) of lead is about 0.6 when the energy ofthe gamma ray is 1.5 MeV. Therefore the radioactive ray shieldingmaterial composed of the high specific-gravity EPDM compositioncontaining tungsten powder having high radioactive ray shieldingperformance has high radioactive ray shielding performance incorrespondence to the mixing rate of the tungsten.

[0083] More specifically, the radioactive ray shielding material of thepresent invention can be used by embedding it in a required portion orwinding it around the required portion as a sealing material for a pipeor a wall of an atomic power plant and a panel of the atomic power plantwhen a repair work is performed. The radioactive ray shielding materialcan be also used as a guard material surrounding the periphery of anX-ray inspection machine that is used to inspect foreign matters mixedin food and inspect baggage at a customs house. The radioactive rayshielding material can be also used in the form of shop curtain havingslits formed on a sheet. The radioactive ray shielding material can bealso used for a syringe, gloves, protection cloths, a material coveringradioactive substances, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

[0084]FIG. 1 is a perspective view showing adynamic damper composed of ahigh specific-gravity EPDM composition of the present invention.

[0085]FIG. 2A is a front view showing the dynamic damper of theembodiment of the present invention.

[0086]FIG. 2B is a side view showing the dynamic damper of theembodiment of the present invention.

[0087]FIG. 2C is a plan view showing the dynamic damper of theembodiment of the present invention.

[0088]FIG. 3 is a perspective view showing a state in which the dynamicdamper of the present invention has been installed on a racket frame.

[0089]FIG. 4 is a plan view showing a tennis racket in which the dynamicdamper of the present invention is installed at three and nine o'clockpositions of the racket frame.

[0090]FIG. 5 is a block diagram showing a system for measuring afrequency and a damping ratio of the tennis racket.

[0091]FIG. 6 is a graph showing the relationship between a frequency anda transmission function in an analysis to be made in system formeasuring a frequency and a damping ratio of the tennis racket.

[0092]FIG. 7 is a schematic view showing a measuring position for afrequency in an out-of-plane secondary mode of the tennis racket.

[0093]FIG. 8 is a schematic view showing a measuring position for afrequency in an in-plane tertiary mode of the tennis racket.

[0094]FIGS. 9A and 9B are an explanatory view respectively forexplaining an out-of-plane secondary mode of the tennis racket.

[0095]FIGS. 10A and 10B are an explanatory view respectively forexplaining an in-plane tertiary mode of the tennis racket.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0096] The embodiments of the present invention will be described belowwith reference to drawings.

[0097] A high specific-gravity EPDM composition of a first embodiment ofthe present invention contains EPDM containing at 4.0 wt % of diene and66 wt % of ethylene. The EPDM has a Mooney viscosity 165 at 125° C.

[0098] A base material is obtained by mixing 200 wt % of the EPDM, 100wt % of oil serving as a softener, a required wt % of powdery sulfur, avulcanizing accelerator, carbon, zinc oxide, stearic acid, and an ageresister and then kneading them by an enclosed-type kneader.

[0099] Tungsten powder whose average diameter is 9 μm and specificgravity is 19.3 is added in an amount of 400 g to 25 g of the basematerial without surface-treating the tungsten powder. Then the basematerial and the tungsten powder are kneaded by the enclosed-typekneader to obtain the high specific-gravity EPDM composition. The weightratio of the tungsten to the high specific-gravity EPDM composition is94.1%. The specific gravity of the high specific-gravity EPDMcomposition is 9.2.

[0100] The X-ray absorption characteristic of the high specific-gravityEPDM composition at 6 MeV is about 96% of that of a lead plate having athickness equal to that of the high specific-gravity EPDM compositionand about twice as large as that of a commercially available lead sheet(specific gravity: 4) having a thickness equal to that of the highspecific-gravity EPDM composition. In this way, the highspecific-gravity EPDM composition has radioactive ray shieldingperformance almost equal to that of lead or superior to that of alead-containing sheet.

[0101] The surface hardness of the vulcanized high specific-gravity EPDMcomposition measured by the method specified by JIS K-6253 (testerdurometer type A) is 72, and the tensile strength of the vulcanized highspecific-gravity EPDM composition is 5.1 MPa.

[0102]FIGS. 1 through 3 show a dynamic damper 10 according to thepresent invention. The high specific-gravity EPDM composition is used asa mass-adding part 11 of the dynamic damper 10 .

[0103] As shown in FIG. 1, the dynamic damper is composed of a sheetincluding the sheet-shaped mass-adding part 11 and a sheet-shapedviscoelastic part 12 layered thereon and integral therewith. Threehorizontal frames 13 formed by bending the above-described sheet in theshape of “U” in section are disposed almost parallel to one another atcertain intervals. Two vertical frame 14, consisting of theabove-described sheet, parallel to each other are positioned at bothends of each of the horizontal frames 13. Thus the dynamic damper 10 islattice-shaped.

[0104] The high specific-gravity EPDM composition is used for themass-adding part 11. A rubber material containing the EPDM as its maincomponent is used for the viscoelastic part 12.

[0105] The total of the thickness of the mass-adding part 11 and that ofthe viscoelastic part 12 is set to the range of 2.8 mm to 7.5 mm. In theembodiment, the total thickness of both parts 11 and 12 is 4 mm. Thethickness of the mass-adding part 11 and that of the viscoelastic part12 are 0.6 mm and 3.4 mm respectively.

[0106] As shown in FIGS. 2A, 2B, and 2C, in the dynamic damper 10, thewidth W1 of the U-shaped horizontal frame 13 of the dynamic damper 10 is5 mm. The interval W2 between the adjacent horizontal frames 13 is 5.5mm. The length L2 of the long narrow vertical frame 14 is 26 mm. Thelength L1(length vertical to ball-hitting face when dynamic damper ismounted on tennis racket) of the U-shaped horizontal frame 13 is 41 mm.

[0107] Supposing that the top position of the ball-hitting face F of theracket frame 2 is 12 o'clock by regarding the ball-hitting face Fsurrounded with a head part 3 as the face of a clock, as shown in FIGS.3 and 4, the dynamic damper 10 is installed at the three and nineo'clock positions of the racket frame 2. The dynamic damper 10 isinstalled on the racket frame in such a way that the longitudinaldirection of the vertical frame 14 of the dynamic damper 10 is disposedparallel to that of the racket frame 2.

[0108] More specifically, as shown in FIG. 3, the dynamic damper 10 isinstalled on the racket frame 2, with the central portion of theU-shaped horizontal frame 13 disposed on the inner surface of the racketframe 2 in its thickness direction, the bent portion of the horizontalframe 13 disposed at both sides thereof disposed on both faces of theracket frame 2 in its widthwise direction, the long and narrow verticalframe 14 disposed on both faces of the racket frame 2 in its widthwisedirection, and the surface of the dynamic damper 10 at the side of theviscoelastic part 12 thereof in contact with the surface of the innerside (gut-stretched side)of the racket frame 2. The three horizontalframes 13 parallel with one another are installed on the racket frame 2,with the horizontal frames 13 sandwiching gut insertion holes gtherebetween.

[0109] As shown in FIG. 4, the racket frame 2 of the tennis racket 1 iscomposed of the head part 3 surrounding the ball-hitting face F, athroat part 4, a shaft part 5, and a grip part 6. These parts arecontinuously formed. A yoke 7 separate from the head part 3 iscontinuous with a throat part of the racket frame 2. Thus the head part3 and the yoke 7 surround the ball-hitting face F annularly.

[0110] The racket frame 2 is composed of a hollow pipe made of a fiberreinforced resin. More specifically, the pipe is made of a laminate offiber reinforced prepregs in which carbon fibers are impregnated withepoxy resin serving as the matrix resin.

[0111] In the embodiment, the entire length of the tennis racket 1 isset to 699 mm, as shown in FIG. 4. The thickness of the head part 3surrounding the ball-hitting face F is set to 24 mm. The thickness ofthe throat part 4 is set to 21 mm. The width of the head part 3 is setto 12 mm. The width of the throat part 4 is set to 14 mm. The thicknessand width of the portion of the racket frame 2 on which the dynamicdamper 10 is installed are set to 21 mm and 12 mm respectively. Thethickness and width of the portion of the racket frame 2 at both sidesof each dynamic damper-installing portion are set to 24 mm and 14.5 mmrespectively which are a little thicker than the thickness and width ofthe dynamic damper-installing portion respectively.

[0112] As described above, since the dynamic damper that is mounted onthe tennis racket is composed of the high specific-gravity EPDMcomposition used as the mass-adding part, it is possible to make thevolume and thickness of the dynamic damper small. Therefore a player canplay tennis without caring about the presence of the dynamic damper.Further a small air resistance acts on the dynamic damper, which allowsthe player to have high operability in using a tennis racket. Althoughthe dynamic damper is smaller than the conventional one, the formerprovides sufficient vibration-damping performance.

[0113] The dynamic damper of the present invention is manufactured inthe following process:

[0114] Initially, the high specific-gravity EPDM composition issufficiently kneaded. Thereafter, it is heated under a pressure to shapeit into a sheet. Thereafter, the sheet is cut to a necessary size toobtain a mixture piece for the mass-adding part of the dynamic damper.Then the obtained mixture piece is set in a die having a desiredconfiguration. Then a material for the viscoelastic part is filled intothe die. Then the mixture for the mass-adding part and the material forthe viscoelastic part are pressed and heated. As a result, both arebonded to each other by vulcanization to obtain a sheet-shaped dynamicdamper composed of the mass-adding part and the viscoelastic partdisposed thereon.

[0115] Instead of the above-described method, it is possible to set thematerial kneaded by a mill and set it into a cavity of a die for themass-adding part. The material is shaped by press molding at a certaintemperature to obtain the material for the mass-adding part. Then thematerial for the mass-adding part is set in the die for the dynamicdamper.

[0116] In the embodiment, the dynamic damper is mounted at the threeo'clock position and the nine o'clock position of the head part of theracket frame. However, the dynamic damper may be installed on at leastone portion of the head part, of the racket frame, surrounding theball-hitting face or/and at least one portion of the throat part of theracket frame.

[0117] The composition of the racket frame is fiber-reinforced resin ormetal or the like, but it is not limited to them. The dynamic damper maybe applied to all the kind of tennis racket.

[0118] In this embodiment, since the number of the horizontal frames isthree, it has the shape of a Japanese character “

”. The number of the horizontal frames may be two. In this case, thedynamic damper is rectangular. In the case where the dynamic damper hasfour horizontal frames, it has the shape of a Japanese character “

”. So long as the dynamic damper is composed of the mass-dding part andthe viscoelastic part, needless to say, its shape is not limited to alattice.

[0119] Examples 1-7 of the high specific-gravity EPDM composition of thepresent invention and comparison examples 1-5 will be described indetail below.

[0120] The EPDM and other components such as additives were mixed witheach other at weight percentages shown in table 1. As will be describedbelow, different kinds of the EPDM were used in the examples and thecomparison examples. The mixing amount of a softener and that oftungsten will be described later. Unit in table1 was parts by weight.TABLE 1 Component Parts by weight EPDM 200 HAF (carbon) 40 Zinc oxide(two kinds) 5 Stearic acid 1 Age register IRGANOX 1010 0.5 IRGANOXMD1024 0.5 Powdery sulfur 1 Vulcanization accelerator M 1 Vulcanizationaccelerator TET 0.5 Vulcanization accelerator BZ 0.5 Vulcanizationaccelerator TTTE 0.5

[0121] As will be described later, oil was added to the components ofeach of the examples and the comparison examples as the softener byvarying the kind and amount thereof. The components of each of theexamples and the comparison examples were kneaded by a compactenclosed-type kneader (Mixlabo SW produced by Moriyama Co., Ltd.).

[0122] An amount of 400 g of tungsten powder (SG50-W produced by TokyoTungsten Co., Ltd.) whose average diameter was 9 μm and specific gravitywas 19.3 was added to 25 g of the base material in a powdered state,namely, without surface-treating the tungsten powder with a couplingagent. Then the base material and the tungsten powder were kneaded bythe enclosed-type kneader to obtain the high specific-gravity EPDMcomposition. The weight ratio of the tungsten powder to the highspecific-gravity EPDM composition was 94.1%.

[0123] Then the high specific-gravity EPDM composition was set in a dieto press it at 170° C. for 15 minutes to obtain a sheet having athickness of 0.5 mm.

EXAMPLE 1

[0124] As the EPDM, Esprene 670F produced by Sumitomo Chemical Co., Ltd.was used. The amount of diene was 4.0 wt %. The amount of ethylene was66 wt %. The Mooney viscosity of the EPDM at 125° C. was 165. TheEsprene 670F contained 100 wt % of oil for 100 wt % of the EPDM. Thespecific gravity of the high specific-gravity EPDM composition was nine.

EXAMPLE 2

[0125] As the EPDM, Esprene 512F produced by Sumitomo Chemical Co., Ltd.was used. The amount of diene was 4.0 wt %. The amount of ethylene was65 wt %. The Mooney viscosity of the EPDM at 125° C. was 66. As asoftener, 30 wt % of oil (Diana Process Oil PW380 produced by IdemitsuKosan Co., Ltd.) was added to 100 wt % of the EPDM. The specific gravityof the high specific-gravity EPDM composition was nine.

EXAMPLE 3

[0126] As the EPDM, Esprene 601F produced by Sumitomo Chemical Co., Ltd.was used. The amount of diene was 3.5 wt %. The amount of ethylene was59 wt %. The Mooney viscosity of the EPDM at 125° C. was 160. TheEsprene 601F contained 70 wt % of oil for 100 wt % of the EPDM. Thespecific gravity of the high specific-gravity EPDM composition was nine.

EXAMPLE 4

[0127] As the EPDM, Esprene 673 produced by Sumitomo Chemical Co., Ltd.was used. The amount of diene was 4.5 wt %. The amount of ethylene was64 wt %. The Mooney viscosity of the EPDM at 125° C. was 110. As asoftener, 40 wt % of oil (Diana Process Oil PW380 produced by IdemitsuKosan Co., Ltd.) was added to 100 wt % of the EPDM. The specific gravityof the high specific-gravity EPDM composition was nine.

EXAMPLE5

[0128] As the EPDM, Esprene 533 produced by Sumitomo Chemical Co., Ltd.was used. The amount of diene was 4.5 wt %. The amount of ethylene was58 wt %. The Mooney viscosity of the EPDM at 125° C. was 100. As asoftener, 40 wt % of oil (Diana Process Oil PW380produced by IdemitsuKosan Co., Ltd.) was added to 100 wt % of the EPDM. The specific gravityof the high specific-gravity EPDM composition was nine.

EXAMPLE 6

[0129] As the EPDM, Esprene 512F was used similarly to the example 5 1.As a softener, 50 wt % of oil (Diana Process Oil PW380 produced byIdemitsu Kosan Co., Ltd.) was added to 100 wt % of the EPDM. Thespecific gravity of the high specific-gravity EPDM composition was nine.

COMPARISON EXAMPLE 1

[0130] As the EPDM, Esprene 522 produced by Sumitomo Chemical Co., Ltd.was used. The amount of diene was 5.0 wt %. The amount of ethylene was56 wt %. The Mooney viscosity of the EPDM at 125° C. was 58. Oil was notadded to the EPDM. The specific gravity of the high specific-gravityEPDM composition was nine.

COMPARISON EXAMPLE 2

[0131] As the EPDM, Esprene 582F produced by Sumitomo Chemical Co., Ltd.was used. The amount of diene was 6.0 wt %. The amount of ethylene was71 wt %. The Mooney viscosity of the EPDM at 125° C. was 67. As asoftener, 30 wt % of oil (Diana Process oil PW380 produced by IdemitsuKosan Co., Ltd.) was added to 100 wt % of the EPDM. The specific gravityof the high specific-gravity EPDM composition was nine.

COMPARISON EXAMPLE 3

[0132] As the EPDM, Esprene 524 produced by Sumitomo Chemical Co., Ltd.was used. The amount of diene was 4.5 wt %. The amount of ethylene was63 wt %. The Mooney viscosity of the EPDM at 125° C. was 25. Oil was notadded to the EPDM. The specific gravity of he high specific-gravity EPDMcomposition was nine.

COMPARISON EXAMPLE 4

[0133] As the EPDM, Esprene 505A produced by Sumitomo Chemical Co., Ltd.was used. The amount of diene was 9.5 wt %. The amount of ethylene was50 wt %. The Mooney viscosity of the EPDM at 125° C. was 34. Oil was notadded to the EPDM. The specific gravity of he high specific-gravity EPDMcomposition was nine.

EXAMPLE 7

[0134] As the EPDM, Esprene 670F produced by Sumitomo Chemical Co., Ltd.was used. The amount of diene was 4 wt %. The amount of ethylene was 66wt %. The Mooney viscosity of the EPDM at 125° C. was 165. As asoftener, 170 wt % of oil (Diana Process Oil PW380 produced by IdemitsuKosan Co., Ltd.) was added to 100 wt % of the EPDM. The specific gravityof the high specific-gravity EPDM composition was nine.

COMPARISON EXAMPLE 5

[0135] As tungsten-containing chloroprene comumercially available. aheavy metal sheet (HMS-09C produced by Sumitomo Electric Industries,Ltd.) was used.

[0136] The hardness (standard A) of the sheet (sheet could not be formedin the comparison example 4) of the examples 1-7 and the comparisonexamples 1-5 was measured. The tensile strength and tensile elongationof each sheet were also measured in the tensile test. A weatherabilitytest was also conducted to evaluate whether blooming occurred on thesheets. The test method and the evaluation method will be describedlater. Table 2 shown below indicates the kind of the EPDM, the kind ofthe EPDM, components thereof, and results of evaluations. TABLE 2 E1 E2E3 E4 E5 E6 EPDM (Esprene 670F 512F 60W 673 553 670F number) Dieneamount (wt %) 4 4 3.5 4.5 4.5 4 Ethylene 66 65 59 64 58 66 amount (wt %)Mooney 165 66 160 110 100 165 viscosity (125° C.) Oil amount (wt %) 10030 70 80 40 150 Hardness (A) 72 85 84 85 80 54 Tensile 5.1 9.7 8.4 9.49.59 3.84 strength (MPa) Tensile 590 195 681 170 190 609 elongation (%)Blooming ◯ ◯ ◯ ◯ ◯ ◯ CE1 CE2 CE3 CE4 E7 CE5 EPDM (Esprene 522 582F 524505A 670F — number) Diene amount (wt %) 5 6 4.5 9.5 4 — Ethylene 56 7163 50 66 — amount (wt %) Mooney 58 67 25 34 165 — viscosity (125° C.)Oil amount (wt %) 0 30 0 0 170 — Hardness (A) 70 88 77 — 39 88 Tensile2.6 10.6 2.8 — 2.7 4.3 strength (MPa) Tensile 350 200 320 — 605 530elongation (%) Blooming X X ◯ X Δ Δ

[0137] Measurement of Hardness

[0138] The surface hardness of each vulcanized high specific-gravityEPDM composition was measured by the method specified by JIS K-6253(tester durometer type A).

[0139] Tensile Test

[0140] In JIS 3 dumbbell shape, a tensile test was conducted on thespecimen sheets at a tensile speed of 500 mm/min to measure the strengthand elongation thereof at the time of breakage thereof.

[0141] Weatherability Test

[0142] A weatherability test was conducted by a sunshine super-longweatherometer (WEL-SUN-HC•B type produced by Suga tester Co., Ltd.) for120 hours to check whether each specimen sheet had blooming or not.

[0143] In the examples 1-6, the EPDM whose diene amount and ethyleneamount were in the specified range was used, and the oil was used in thespecified range. Therefore the surface hardness of the vulcanizedspecimen sheet was less than 90, and the tensile strength thereof wasnot less than 3 MPa. Thus the specimen sheet did not bloom and had goodweatherability. It was confirmed that the specimen sheet of the examples1-6 was optimum as the material for the mass-adding part of the dynamicdamper.

[0144] In the example 7, the same EPDM as that of the examples 1 and 6was used. However since a comparatively large amount of oil serving asthe softener was used, the specimen sheet had a lower strength and hadmore blooming than the examples 1-6, but the extent of the blooming hasno problem in the use thereof. Thus it was confirmed the specimen sheetof the example 7 was also suitable as the material for the mass-addingpart of the dynamic damper.

[0145] In the comparison example 1, since the amount of ethylene wasless than 58 wt %, the strength of the specimen sheet was low. Since theamount of diene was more than 4.5 wt %, the specimen sheet had bloomingand low weatherability.

[0146] In the comparison example 2, although the specimen sheet had ahigh strength, the diene amount was more than that of the comparisonexample 1. Thus the specimen sheet bloomed more than the specimen sheetof the comparison example 1 and had low weatherability.

[0147] Since the EPDM of the comparison example 3 had a low Mooneyviscosity, the specimen sheet had a low strength.

[0148] Since the EPDM of the comparison example 4 had a small amount ofethylene and a low Mooney viscosity, the moldability of the specimensheet was so low that the specimen sheet was broken into pieces andcould not be processed into a sheet. In the weatherability test of abroken piece, it had blooming and had a poor weatherability.

[0149] The specimen sheet of the comparison example 5 had no problem inits hardness and strength. However, in the weatherability test, thesurface thereof deteriorated and had blooming and inferiorweatherability.

[0150] From the above, it can be confirmed that by using the EPDM havingan optimum condition, adding an optimum amount of softener to the EPDM,and using a specified amount of tungsten powder, it is possible to formthe high specific-gravity material that was appropriately soft as thematerial for the mass-adding part of the dynamic damper, can beprocessed in combination with other materials, had no problem instrength, and did not have blooming.

[0151] The example 8 in which the dynamic damper is composed of the highspecific-gravity EPDM composition of the present invention and thecomparison example 6 will be described in detail below.

EXAMPLE 8

[0152] As the material for the mass-adding part, the sheet prepared inthe example 1 was used.

[0153] As the material for the viscoelastic part, the material composedof the components shown in table 3 was used. The components were kneadedwith an enclosed-type kneader. TABLE 3 Component Parts by weight Esprene532 (EPDM) (produced by Sumitomo 100 Chemical Co., Ltd.) Diana processoil Px-90 (produced by Idemitsu 250 Kosan Co., Ltd.) Zinc oxide (twokinds) 150 Stearic acid 5 Powdery sulfur 1 Vulcanization accelerator M1.0 Vulcanization accelerator TET 0.5 Vulcanization accelerator BZ 0.5Vulcanization accelerator TTTE 0.5 Titanium oxide 10

[0154] The mass-adding part and the viscoelastic part are layered oneach other and set in a die. Thereafter the laminate was pressed at 170°C. for 20 minutes and vulcanized into the shape of a dynamic damper. Theshape of the dynamic damper was similar to that of the example 1.

[0155] Comparison Example 6

[0156] As the mass-adding part, a heavy metal sheet (produced bySumitomo Electric Industries, Ltd.) having a thickness of 0.6 mm wasused. The heavy metal sheet was made of tungsten-containing chloroprenerubber. The viscoelastic part was similar to that of the example 8. Adynamic damper having the same configuration as that of the example 7was prepared by a method similar to the above-described method.

[0157] The dynamic damper of the example 8 and that of the comparisonexample 6 were installed at three and nine o'clock positions of the headpart of the tennis racket respectively. The out-of-plane secondarynatural frequency, damping ratio, in-plane tertiary natural frequency,and damping ratio of the dynamic damper-mounted tennis racket of each ofthe example 8 and the comparison example 6 were measured. Theweatherability of each dynamic damper was evaluated by conducting an outdoor exposure test. The test method and the measuring method will bedescribed later. Table 4 shows results of the evaluation. TABLE 4Out-of-plane Damp- In-plane Damp- secondary ing tertiary ing Outdoornatural ratio natural ratio exposure frequency (Hz) (%) frequency (Hz)(%) test E8 422 4.5 371 5.5 ◯ CE6 421 4.4 370 5.3 Δ Blooming occurred

[0158] Measurement of Natural Frequency and Damping Ratio

[0159] The method of measuring the natural frequency of each of thetennis rackets TR and the damping ratios thereof is shown in FIGS. 5 and6. To measure them with high accuracy, an acceleration pick-up meter 73was mounted on a maximum amplitude position of the tennis racket TR ineach vibration mode. In this state, the maximum amplitude position ofthe tennis racket TR was hit with an impact hammer 71 to impartvibrations to the tennis racket TR. No gut was stretched on thegut-stretched part of the racket frame. As shown in FIGS. 7 and 8, thenatural frequency of the tennis racket TR and its damping ratio weremeasured by a free supporting method of hanging the tennis racket TRwith a string. An input vibration (F) measured with a force pick-upmeter installed on the impact hammer 71 and a response vibration (a)measured with the acceleration pick-up meter 73 were inputted to afrequency analyzer 74 (manufactured by Furet Packard Corp., dynamicsingle analyzer HP 3562A) through amplifiers 72 and 70 to analyze theinput vibration (F) and the response vibration (α). This method wascarried out by supposing that the rigidity of the racket frame waslinear. Table 4 shows the results of measurement on tennis racket ofeach of the examples and comparison examples.

[0160] A transmission function, in a frequency region, obtained by theanalysis was determined to obtain the out-of-plane secondary frequencyand the in-plane tertiary natural frequency of the racket frame. Thevibration-damping ratio (ζ) was computed with reference to FIG. 6 byusing the following equation:

ζ=({fraction (1/2)})×Δω/ω n)

To=Tn/{square root}2

[0161] Measurement of Out-of-plane Secondary Natural Frequency

[0162] As shown in FIG. 7, the out-of-plane secondary natural frequencyis a second peak which appears with respect to a low frequency when thetennis racket 1 set in a free supporting state of hanging the tennisracket 1 with a string is hit on its back. More specifically, theout-of-plane secondary frequency is a frequency at the time when thetennis racket 1 (before deformation) shown in FIG. 9A vibrates in theout-of-plane secondary mode, as shown in FIG. 9B (side view of thetennis racket).

[0163] Measurement of In-plane Tertiary Natural Frequency

[0164] As shown in FIG. 8, the in-plane tertiary natural frequency is athird peak which appears with respect to the low frequency when thetennis racket 1 set in a free supporting state of hanging the tennisracket 1 with a string is hit from the outside. More specifically, thein-plane tertiary natural frequency is a frequency (before deformation),shown in FIG. 10A, at the time when the tennis racket 1 vibrates(deforms) in the in-plane tertiary mode, as shown in FIG. 10B.

[0165] Outdoor Exposure Test

[0166] An outdoor exposure test was conducted on the tennis racket onwhich the dynamic damper of the example 8 was mounted and the tennisracket on which the dynamic damper of the comparison example 6 wasmounted. In the outdoor exposure test, the tennis rackets were exposedto rain, wind, and sunshine for two months of August and September.Table 4 shows the result of evaluation.

[0167] As described above, the mass-adding part of the dynamic damper ofthe example 8 was composed of the high specific-gravity EPDMcomposition, and the mass-adding part of the dynamic damper of thecomparison example 6 was composed of the tungsten-containing chloroprenerubber commercially available. The vibration-damping performance of thedynamic damper of the example 8 was equal to that of the dynamic damperof the comparison example 6. As a result of exposure to the scorchingheat of the sun in two months in summer, the dynamic damper of theexample 8 did not have blooming and was superior in weatherability,whereas the dynamic damper of the comparison example 6 deteriorated alittle on its surface. It can be confirmed that the dynamic dampercomposed of the high specific-gravity EPDM composition of the presentinvention is superior in weatherability and in vibration-dampingperformance.

[0168] Evaluation was made on the performance of the radioactive rayshielding material composed of the high specific-gravity EPDMcomposition of each of the examples 1-7. To do so, using the highspecific-gravity EPDM compositions (specific grdvity. nine) of theexamples 1-7, sheets having a thickness of 1 mm were prepared.

[0169] As a comparison example 7, a lead plate in the same configurationas that of the examples 1-7was used. As a comparison example 8, alead-containing sheet (specificgravity: four) having the sameconfiguration as that of the examples 1-7 was used.

[0170] Measurement of Radioactive Ray Shielding Performance

[0171] The radioactive ray absorption characteristics of the sheetscomposed of the high specific-gravity EPDM composition of the examples1-7 respectively, the lead plate of the comparison example 7, and thelead-containing sheet of the comparison example 8 were measured byirradiating them with X-rays of 6 MeV.

[0172] The result of the measurement was that the radioactive rayshielding performance of the sheet of each of the examples 1-7 was 95%of that of the lead plate of the comparison example 7 and 1.9 times aslarge as that of the lead-containing sheet of the comparison example 8.It can be confirmed that the radioactive ray shielding performance ofthe sheet of each of the examples 1-7 is almost equal to or higher thanthat of the conventional lead plate or the conventional lead-containingsheet.

[0173] As apparent from the foregoing description, the present inventionprovides the high specific-gravity EPDM composition which is the mixtureof the EPDM containing diene and ethylene at a wt % in the specifiedrange and having a Mooney viscosity in the specified range at 125° C.and the powdery material containing powder whose specific gravity is notless than 12 as a main component thereof. The powdery material is addedto the EPDM at not less than 80 wt % nor more than 97.5 wt % of thewhole amount (total weight of EPDM, additives, and powdery material) ofthe high specific-gravity EPDM composition. Thus the highspecific-gravity EPDM composition does not have blooming and hasimproved weatherability. Further the high specific-gravity EPDMcomposition is high in moldability and processability.

[0174] Further the high specific-gravity EPDM composition is soft, has ahigh specific gravity and strength, and does not pollute environment.Therefore the high specific-gravity EPDM composition can be used for thedynamic damper, the radioactive ray shielding material, thevibration-damping/sound insulation sheet, the soundproof material, andthe like.

[0175] Further the high specific-gravity EPDM composition isappropriately soft, can be processed together with other materials, anddoes not have any problems in strength nor blooming. Therefore the highspecific-gravity EPDM composition can be preferably used as the materialfor the mass-adding part that is layered on the viscoelasticpart.Moreover since the high specific-gravity EPDM composition has a highspecific gravity, it is possible to make the volume and thickness of thedynamic damper small. Thus the dynamic damper does not disturb a playerduring the use of a tennis racket and is unnoticeable in appearance.Further since the dynamic damper is soft, there is no fear that theplayer is injured thereby, even when the player touches the dynamicdamper or strikes it against others by mistake.

[0176] Further since it is possible to form the dynamic damper as a thinsheet, a player can play tennis without caring about the dynamic damper.Furthermore since a small air resistance acts on the dynamic damper, theplayer has high operability in using the tennis racket. Although thedynamic damper is smaller than the conventional one, the former providessufficient vibration-damping performance.

[0177] The radioactive ray shielding material of the present inventiondoes not pollute environment and has high moldability, processability,heat resistance, and strength, and radioactive ray shieldingperformance. Thus the radioactive ray shielding material can be easilyprocessed into various configurations and preferably used as areplacement of metal and for radiation therapy, atomic power plants, andindustrial and medical radioactive ray inspection machines.

What is claimed is:
 1. A high specific-gravity EPDM compositionconsisting of a mixture of EPDM containing diene at less than 4.5 wt %and ethylene at not less than 58 wt % nor more than 80 wt % and having aMooney viscosity ML₁₊₄ not less than 50 nor more than 170 at 125° C. anda powdery material, containing powder whose specific gravity is not lessthan 12 as a main component thereof, added to said EPDM at not less than80 wt % nor more than 97.5 wt % of a whole amount of said highspecific-gravity EPDM composition.
 2. The high specific-gravity EPDMcomposition according to claim 1, wherein less than 150 wt % of asoftener is added to 100 wt % of said EPDM.
 3. The high specific-gravityEPDM composition according to claim 1 or 2, wherein said powderymaterial is tungsten, a tungsten compound or a tungsten based alloy. 4.The high specific-gravity EPDM composition, according to any one ofclaims 1 through 3, having a specific gravity not less than 4.5 nor morethan 13.1.
 5. The high specific-gravity EPDM composition according toany one of claims 1 through 4, wherein a surface hardness of saidvulcanized high specific-gravity EPDM composition measured by a methodspecified by JIS K-6253 (tester type A) is less than 90; and a tensilestrength of said vulcanized high specific-gravity EPDM composition isnot less than 3 MPa.
 6. A dynamic damper composed of a viscoelastic partand a mass-adding part, wherein a high specific-gravity EPDM compositionaccording to any one of claims 1 through 5 is used as saidmass-addingpart.
 7. A tennis racket installing a dynamic damper according to claim6 on at least one portion of a head part surrounding a ball-hitting faceof a racket frame or/and at least one portion of a throat part thereof.8. The tennis racket according to claim 7, wherein a mass-adding part iscomposed of said high specific-gravity EPDM composition molded in ashape of a sheet and layered on said viscoelastic part and integratedtherewith.
 9. The tennis racket according to claim 8, wherein athickness of said sheet-shaped mass-adding part is set to not less than0.3 mm nor more than 1.7 mm.
 10. A radiation-shielding materialcomprising a high specific-gravity EPDM composition according to any oneof claims 1 through 5 .